A variety of chromium containing aqueous solutions have heretofore been used or proposed for treating zinc, zinc alloy, cadmium and cadmium alloy surfaces for improving the corrosion resistance properties thereof and to further enhance the appearance of such surfaces by imparting a yellow or a blue-bright coating thereto, the latter simulating a chromium finish. Such treating solutions originally contained chromium in the hexavalent state and in more recent years the chromium constituent was present as a mixture of the hexavalent and trivalent forms. The reduced toxicity of trivalent chromium and the increased simplicity and efficiency in treating waste effluents containing trivalent chromium has occasioned an increased commercial use of treating solutions in which the chromium constituent is substantially entirely in the trivalent state. Such prior art trivalent chromium passivating solutions have been found to be somewhat less effective than the hexavalent chromium passivate solutions in imparting good corrosion resistance to the zinc, zinc alloy, cadmium and cadmium alloy surfaces treated and there has, accordingly, been a continuing development of further improvements in trivalent chromium passivate treatment solutions and processes.
The foregoing problem has been further aggravated by a conversion from conventional cyanide zinc and cadmium plating processes to acid and alkaline non-cyanide electroplating baths which produce metal deposits which are not as receptive to chromium passivate treatments.
Typical of prior art compositions and processes for treating zinc and zinc alloy surfaces are those disclosed in U.S. Pat. Nos. 2,393,663; 2,559,878; 3,090,710; 3,553,034; 3,755,018; 3,795,549; 3,880,772; 3,932,198; 4,126,490; 4,171,231; British Pat. Nos. 586,517 and 1,461,244; and German Pat. No. 2,526,832.
While improvements have been made in trivalent chromium passivate compositions and processes to produce commercially acceptable passivate films, a continuing problem associated with such operating baths has been the relatively rapid loss of the peroxide-type oxidizing agent, particularly hydrogen peroxide, which is present as a necessary bath constituent to achieve acceptable passivate films. Such prior art operating baths also undergo a relatively rapid rise in pH necessitating careful control and addition of acids to maintain the pH level within the optimum operating range. The progressive loss of the peroxide-type oxidizing agent, particularly hydrogen peroxide, is due in part to the presence of actiavting metal ions present in the solution as well as contaminating metal ions such as zinc or cadmium, for example, introduced by dissolution of the metal from the substrates being treated which tend to catalyze a decomposition of the peroxide oxidizing agent. The progressive loss of the peroxide-type oxidizing agents occurs not only during processing but also during standing of the bath overnight and over weekends during plant shutdown. Typically, a fresh operating bath containing 3% by volume of a 35% solution of hydrogen peroxide on standing overnight will lose about 0.1% by volume per hour of the hydrogen peroxide oxidizing agent while a used solution containing from about 2 to about 10 grams per liter of contaminating zinc ions will experience a loss of hydrogen peroxide at a rate as great as about 0.4% by volume per hour. It will be apparent from the foregoing that careful monitoring of the operating bath composition and frequent replenishment of the peroxide oxidizing agent is required to maintain optimum bath efficiency which is not only costly but also time consuming.
The present invention provides a treating solution and process which is effective to impart improved corrosion resistance to zinc, zinc alloy, cadmium and cadmium alloy, as well as aluminum and magnesium surfaces and to impart a desirable surface finish which can range from a clear bright to a light blue-bright to a yellow iridescent appearance, which produces a passivate film of improved corrosion resistance, hardness, durability, clarity and initial hardness, which provides a treating solution that is stabilized against rapid loss of the peroxide oxidizing agent and against a rapid rise in pH, which process is simple to control and operate and which is of efficient and economical operation.